Large storage tanks are often cylindrical and have a circular floating roof. The roof floats on the surface of the liquid, thereby decreasing the vapor space inside of the tank. A floating roof may be required for reasons of safety or for pollution reduction. The floating roof has a seal to seal it to the wall of the tank that helps to prevent the escape of the contained liquid or vapors from that liquid.
There are two broad types of storage tanks that utilize floating roofs: tanks having an exposed floating roof and tanks having a fixed roof covering the floating roof. An advantage of the covered tank is that it protects the floating roof from undesirable effects from the external environment, such as rain or birds. A disadvantage of the covered tank is that volatile, explosive, corrosive, or toxic gasses or liquids can accumulate between the floating roof and the fixed roof.
When the storage tank is being filled, it is critically important to ensure that the level of the roof does not exceed the maximum level that is safe for that tank since exceeding that level can damage seals or cause the release of tank contents. The overfilling of tanks is the leading cause of tank-related disasters, such as fires. To prevent such disasters, safety-related organizations such as the American Petroleum Institute mandate the monitoring of tank roofs.
There are several existing methods for monitoring the limits of the tank level, such as a mechanical limit switch or a rod and plunger with a magnetic reed switch (ref. American Petroleum Institute 2350, Overfill Protection for Storage Tanks in Petroleum Facilities, Third Edition). All of these methods require that the monitoring device be affixed to the cover of the tank.
The level of any liquid on the roof of the tank is also of interest. With covered tanks, any liquid would likely be liquid that has escaped from the sealed interior of the tank whereas with uncovered tanks, the liquid could be rainwater or escaped liquid. If liquid is newly detected or if the liquid level is changing when the tank is being filled or emptied (i.e., when the roof is moving) then the contained liquid is likely escaping through the tank seals. The liquid level sensor is also useful for detecting that the tank has been overfilled.